Debian

Debian is an Operating System which uses the Linux kernel. Although it is usually set up on desktop machines, it is being used on an increasing number of web servers too. The reason is that it is really stable, so a server running it won't have to be restarted for months. Debian is also considered to be one of the most safe Operating Systems nowadays, because security patches are released within days after an issue is discovered. This is possible because thousands of well-trained developers are employed by the Debian project on a voluntary basis and not only do they work on the OS itself, they also provide 24/7 support via special e-mail lists and reply within 20 minutes to any kind of question. Thus, you'll have an outstanding level of support directly from the developers who know their own platform in depth, and not from a third-party support team. Debian is also one of the Linux releases with the highest number of packages - more than 30,000. As the Operating System costs nothing to use, you're able to modify both the core and all the packages in the way you see fit. Furthermore, you won't need to pay any kind of license fees.